Semiconductor manufacturing often involves a series of processes including deposition, photolithography and etching. During the photolithographic process, semiconductor manufacturers often use a photomask to copy an image of an electronic circuit on to a semiconductor wafer. Photomasks come in various sizes and shapes (e.g., a one times photomask or a reticle, which is a photomask that can be shot several times onto a single wafer with a photolithographic tool known as a stepper). Photomasks generally include a quartz blank with a patterned chrome layer deposited on one surface of the quartz blank. This patterned metal layer contains a microscopic image of an electronic circuit.
During the photolithographic process, this image is projected onto a wafer. If the image is projected several times onto a single wafer, the photomask is known as a reticle. As design rules have moved toward smaller and more dense integrated circuit (IC) devices, the quality of the projected image has become increasingly important. A poorly projected image may result in the manufacture of a non-functioning IC device. In many cases, a dust particle resting on the surface of a photomask or reticle, causes sufficient distortion of the projected image to render the IC device non-functioning.
As such, many, if not most, IC manufacturers rely on a dust cover film, which may on occasion be referred to as a pellicle, to keep dust from landing on the surface of the photomask. Typically, the film is attached to a frame, which may be attached to the photomask such that the film is held at a fixed distance from the surface of the photomask. During the photolithographic process, the image of any dust particle resting on the film will be out of focus on the wafers surface. As a result, the probability of a dust induced defect at the wafer surface is reduced.
Ideally, a dust cover film should be invisible to the radiant energy of the photolithographic tool. In order to produce clear, well-defined patterns, the film should preferably transmit nearly 100% of the radiant energy used. Most films, however, do not transmit 100% of the radiant energy. A small percentage of the radiant energy is reflected at the interfaces of the film's surface and air. Generally, the amount of light reflected at the interface of two transparent layers (e.g., air and film) depends upon the refractive index, N, of the two layers. More precisely, the amount of reflection depends upon the difference between the N of the first layer and the N of the second layer. The greater the difference, the more reflection.
The index of refraction, N, for air is approximately 1.0. A dust cover film, on the other hand, may have an N of 1.5 or greater. This difference in index of refraction can result in an average reflection of approximately 8 percent of the light striking the surface at a normal incidence (90 degrees to the face of the film). Moreover, taking into account interference within the film, there may be peak reflections of approximately 16 percent. As a result, considerably less than 100 percent of the radiant energy used during the photolithographic process passes through the dust cover film.
Conventional techniques for combating this problem involve the addition of a film layer having a lower refractive index to a substrate film, which is usually made of nitrocellulose. The additional film layer is often spun onto the substrate film. As such, the substrate film needs to be relatively thick (approximately 1-2 .mu.m) to withstand the forces associated with the spinning process. Moreover, when thinner substrate films are used, damage to the substrate film during the spinning process becomes more likely.
Other conventional solutions include using a single layer of film with a relatively low refractive index. This conventional single layer approach does not allow for producing a dust cover film with a refractive index much below 1.4.